US2012/0263427A and U.S. Pat. No. 7,088,900B disclose an optical fiber having a low Rayleigh scattering loss and a low transmission loss which is a silica glass-based optical fiber that includes a core containing an alkali metal element or alkaline-earth metal element. The above optical fiber is produced by drawing an optical fiber preform that includes a core portion containing an alkali metal element or alkaline-earth metal element. Adding an alkali metal element or alkaline-earth metal element to the core portion of the optical fiber preform enables the viscosity of the core portion to be reduced when the optical fiber preform is drawn and thereby enables the network structure of silica glass to be made uniform. This reduces Rayleigh scattering loss resulting from irregularity in the structure. Hereinafter, alkali metal elements and alkaline-earth metal elements are collectively referred to as “alkali metal element or the like” unless otherwise specified.
A diffusion method is a known method for adding an alkali metal element to silica glass. In the diffusion method, while a vapor of the raw material, such as an alkali metal element or an alkali metal salt, is introduced into a silica glass-based glass pipe, the glass pipe is heated with an external heat source or a plasma is generated inside the glass pipe in order to diffuse the alkali metal element into the inner surface of the glass pipe.
After the alkali metal element has been added to a portion near the inner surface of the glass pipe, the glass pipe is heated in order to reduce the diameter of the glass pipe. Subsequent to the diameter reduction, the inner surface of the glass pipe is etched to a predetermined thickness in order to remove unwanted transition metal elements, such as Ni and Fe, which are added to the inner surface of the glass pipe as impurities simultaneously with the addition of the alkali metal element. The alkali metal element, which has a higher diffusion rate than the transition metal elements, can be retained in the inner surface of the glass pipe even after the surface of the glass pipe has been etched to the predetermined thickness in order to remove the transition metal elements. Subsequent to the etching, the glass pipe is heated to form a solid core rod including a central portion containing the alkali metal element. The core rod containing an alkali metal element and glass deposited on the outer periphery of the core rod may be used as a core portion of an optical fiber preform. An optical fiber preform is produced by synthesizing a cladding portion having a lower refractive index than the core portion on the outer periphery of the core portion including the core rod containing an alkali metal element. The optical fiber preform is melted in a high-temperature furnace and drawn to form an optical fiber by a publicly known method.